One important criteria for safely operating a motor vehicle is the maintenance of proper pressure within tires of the vehicle. If tire pressure is not maintained, fuel economy is adversely affected and rates of tire wear increase. Additionally, handling of the vehicle can be adversely affected and the potential for abrupt failure of the tire is increased. Hence, it is advisable to monitor the pressure within vehicle tires on a regular basis.
One of the difficulties encountered in such regular tire pressure monitoring is that a significant amount of time is associated with monitoring tire pressure. In particular, a safety cap must be removed from the valve stem (and not misplaced). The gauge must be placed upon the valve stem of the tire. Care must be taken to make sure that a significant amount of air is not lost from the tire during this gauge installation process. The pressure can then be read from the gauge. If the tire pressure is adequate, the procedure is completed for that tire and the next tire can be checked. On a vehicle with multiple tires, this basic routine can be significantly time consuming, decreasing the likelihood that tire pressure monitoring will occur on a sufficiently frequent basis.
Compounding this difficulty is the inconvenience encountered when tire pressure is low and air needs to be added to the tire. First the gauge is removed. Then the vehicle is brought into proximity with a source of compressed air. Then compressed air is added to the tire. Unless an accurate gauge is associated with the source of compressed air, the source of compressed air must be removed after a relatively short period of time and the pressure gauge reinserted on the valve stem to take another reading. Typically, a half a dozen or so filling steps and measuring steps are involved before the tire pressure has been brought to the proper level.
It is known in the prior art to provide tire pressure gauges which are mountable upon a valve stem and which are sufficiently small and lightweight that they can remain mounted to the tire during operation of the tire. While this solves a portion of the problem, the difficulty associated with adding air to the tire is not alleviated. In at least two prior art patents, tire pressure gauges are taught which further allow for a source of compressed air to pass into the valve stem of the tire while the gauge remains upon the valve stem. In particular, U.S. Pat. No. 5,377,539 to LaSalle and U.S. Pat. No. 4,924,697 to Hunt (each incorporated herein by reference in their entirety) disclose tire pressure gauges which can mount on a valve stem of a tire and which also include a port through which compressed air can be added without removing the tire pressure gauge.
Bourdon tubes are susceptible to de-calibration or other damage if gas having a pressure greater than a maximum pressure for which the Bourdon tube is calibrated is encountered by the Bourdon tube or other pressure sensor. For instance, many Bourdon tubes are calibrated to accurately read pressures between 0 psi (pounds per square inch) and 60 psi. If air or other gases having a pressure greater than 60 psi pass into the Bourdon tube, the Bourdon tube becomes uncoiled beyond an elastic limit of the Bourdon tube so that the Bourdon tube becomes permanently deformed. The Bourdon tube then typically ceases to accurately read pressure through an indicator portion thereof. Other failure modes also exist including rupture of the Bourdon tube so that it does not provide any reading whatsoever thereafter. The problem of Bourdon tube damage and de-calibration is particularly pernicious in that in some over-pressure failure modes, the Bourdon tube may appear to be functioning properly, when in fact it has become de-calibrated.
Many sources of fill air or other fill gas are provided at very high pressures (as high as 250 psi or higher). With such high pressure fill air, tires or other chambers can be quickly filled. However, such high pressure fill air/gas sources provide a serious threat to the proper function of a Bourdon tube, or many other types of pressure sensors. The patents to LaSalle and Hunt fail to address this problem. Accordingly, a need exists for a gauge including an over-pressure protector to protect the pressure sensor from experiencing pressures greater than a maximum pressure for the sensing element.